When a substance is undergoing a change of state, the heat that is been added to the substance at that particular point in time is used to work on the chemical bonds that is holding the molecules of the substance together; the heat is not used to increase the temperature of the substance at that particular point. For instance, in the case of ice, at its melting point the heat supplied is used to break the inter molecular force that is holding the particles of the ice together. When the bonds are completely broken and the ice has to liquid then the temperature of the water will increase.